The present invention relates to so-called scanning projection devices for displaying image information discretely stored, the scanning beam of the projection devices being made up of a plurality of beam components of different light sources, such as a flying spot laser raster scanner. The present invention is used for compensating for the offset between the individual beam components of the scanning beam of such a projection device.
In this case, the image formation is accomplished with the aid of a pulsed light beam, which scans the projection surface, using a particular trajectory, e.g., line by line. In this context, the intensity of this scanning beam is varied according to the image information to be displayed. As already mentioned, in the case of the projection devices in question here, the scanning beam is made up of a plurality of components of different light sources, which cover different wavelength ranges. The beam components ideally overlap in the projection plane and form a single pixel. Thus, by variation of the intensity of the individual light sources, a very large color spectrum may be represented in this manner. However, in practice, an optimal alignment of the light sources with respect to the optical construction of the projection device is rarely attained, which means that generally, an offset between the pixels of the individual beam components is present.
The image information is provided in the form of discrete scanning values. Each scanning value is assigned to an image point. These “ideal” image points are situated in a raster, which covers the entire image surface and is described by integral image coordinates of a rectilinear and, generally, orthogonal image coordinate system. Due to the optical construction, which is necessary for the alignment and deflection of the individual beam components during the scanning/projection operation, the actual trajectory of the scanning beam, that is, of the individual beam components of the scanning beam, deviates from this raster, which means that the position of the projected pixels does not correspond to the raster points. The projected pixels span the so-called projector coordinate system, which, in contrast to the image coordinate system, is not rectilinear and orthogonal, but, as a rule, curved. In this projector coordinate system, the projected pixels are assigned integral projector coordinates.
In a non-corrected pixel video stream, the image information of a particular image point would be assigned to the pixel, whose projector coordinates are identical to the image coordinates of this image point. In this instance, nearly all of the pixels would be projected differently from the corresponding image coordinates. As a result, this procedure produces image distortion.
In order to prevent this, the non-linear difference of the projector coordinates from the image coordinates is compensated for by preprocessing the image information during the projection operation. This preprocessing of the image information requires calibration of the projection device. In this context, a function is determined, which converts the projector coordinates of the pixels to image coordinates, which is referred to as dewarping.
During the projection operation, the projector coordinates of the individual, projected pixels are converted into image coordinates with the aid of this predefined dewarping function, in order to then assign the specific pixel the image information, which corresponds to its position in the image surface. Generally, the image coordinates of the real pixels are not integral. Thus, the regulation of the intensity of the corresponding light source is mostly based on not only a single scanning value of the image information, but on a mean of scanning values, in a surrounding area to be defined, of the image coordinates of the real pixel. This preprocessing of the image information allows the nonlinear difference between image coordinates and projector coordinates to be compensated for, and corresponding image distortion to be prevented.
The complexity and, consequently, the computing expenditure for the dewarping is, first of all, a function of the type of optical layout, which is selected and implemented for the alignment and deflection of the individual beam components during the scanning/projection operation, and secondly, a function of the targeted quality of the image reproduction. If the dewarping is applied to all light sources involved in the projection operation, then not only is the image distortion compensated for, but also the offset between the color components of the individual pixels, which is superimposed on this image distortion. Depending on the number of light sources involved, this is associated with a very high computing expenditure.
PCT Application No. WO 2009/025976 describes an option of compensating for the offset between the individual beam components in the projection plane largely independently of the image distortion. To that end, in an initial calibration step, an offset corresponding to the offset is initially determined for each beam/color component. According to PCT Application No. WO 2009/025976, during the projection operation, the image information is written to a temporary storage unit, in which the image information of an entire image is stored, namely, so as to be divided up according to color channels and provided with position information. In this context, the image information of each color channel, that is, the associated position information, is acted upon by the respective offset determined in advance. The image information acquired in this manner is then used as input data for a dewarping method and a compensation of image distortion based on it. In the case of PCT Application No. WO 2009/025976, the dewarping is therefore only carried out for a single light source involved with the projection operation, which limits the computing expenditure markedly.
However, the variant, described in PCT Application No. WO 2009/025976, of compensating for the offset between the individual beam components of the sampling beam of a projection device requires a relatively large temporary storage unit for the image information. For some applications, such as mobile projection devices from the area of consumer electronics, this proves to be problematic.